1. Field of the Invention
The present invention relates generally to a lithographic projection apparatus and more specifically to a lithographic projection apparatus including radiation level control.
2. Brief Description of the Related Art
The term xe2x80x9cpatterning structurexe2x80x9d as here employed should be broadly interpreted as referring to means that can be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate; the term xe2x80x9clight valvexe2x80x9d can also be used in this context. Generally, the said pattern will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit or other device (see below). Examples of such patterning structure include:
A mask. The concept of a mask is well known in lithography, and it includes mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. Placement of such a mask in the radiation beam causes selective transmission (in the case of a transmissive mask) or reflection (in the case of a reflective mask) of the radiation impinging on the mask, according to the pattern on the mask. In the case of a mask, the support structure will generally be a mask table, which ensures that the mask can be held at a desired position in the incoming radiation beam, and that it can be moved relative to the beam if so desired.
A programmable mirror array. An example of such a device is a matrix-addressable surface having a viscoelastic control layer and a reflective surface. The basic principle behind such an apparatus is that (for example) addressed areas of the reflective surface reflect incident light as diffracted light, whereas unaddressed areas reflect incident light as undiffracted light. Using an appropriate filter, the said undiffracted light can be filtered out of the reflected beam, leaving only the diffracted light behind; in this manner, the beam becomes patterned according to the addressing pattern of the matrix-adressable surface. The required matrix addressing can be performed using suitable electronic means. More information on such mirror arrays can be gleaned, for example, from U.S. Pat. No. 5,296,891 and U.S. Pat. No. 5,523,193, which are incorporated herein by reference. In the case of a programmable mirror array, the said support structure may be embodied as a frame or table, for example, which may be fixed or movable as required.
A programmable LCD array. An example of such a construction is given in U.S. Pat. No. 5,229,872, which is incorporated herein by reference. As above, the support structure in this case may be embodied as a frame or table, for example, which may be fixed or movable as required.
For purposes of simplicity, the rest of this text may, at certain locations, specifically direct itself to examples involving a mask and mask table; however, the general principles discussed in such instances should be seen in the broader context of the patterning structure as hereabove set forth.
Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the patterning structure may generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising one or more dies) on a substrate (silicon wafer) that has been coated with a layer of radiation-sensitive material (resist). In general, a single wafer will contain a whole network of adjacent target portions that are successively irradiated via the projection system, one at a time. In current apparatus, employing patterning by a mask on a mask table, a distinction can be made between two different types of machine. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire mask pattern onto the target portion at once; such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatusxe2x80x94commonly referred to as a step-and-scan apparatusxe2x80x94each target portion is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction (the xe2x80x9cscanningxe2x80x9d direction) while synchronously scanning the substrate table parallel or anti-parallel to this direction; since, in general, the projection system will have a magnification factor M generally  less than 1), the speed V at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. More information with regard to lithographic devices as here described can be gleaned, for example, from U.S. Pat. No. 6,046,792, incorporated herein by reference.
In a manufacturing process using a lithographic projection apparatus, a pattern (e.g. in a mask) is imaged onto a substrate that is at least partially covered by a layer of radiation-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book xe2x80x9cMicrochip Fabrication: A Practical Guide to Semiconductor Processingxe2x80x9d, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4, incorporated herein by reference.
For the sake of simplicity, the projection system may hereinafter be referred to as the xe2x80x9clensxe2x80x9d; however, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, and catadioptric systems, for example. The radiation system may also include components operating according to any of these design types for directing, shaping or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a xe2x80x9clensxe2x80x9d. Further, the lithographic apparatus may be of a type having two or more substrate tables (and/or two or more mask tables). In such xe2x80x9cmultiple stagexe2x80x9d devices the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposures. Twin stage lithographic apparatus are described, for example, in U.S. Pat. No. 5,969,441 and WO 98/40791, incorporated herein by reference.
In a lithographic projection apparatus it is important to ensure that the transmissivity of the radiation system and the projection system for radiation of the projection beam is substantially stable during an exposure of a target portion. This facilitates appropriate control over dose (xe2x80x9cdose controlxe2x80x9d). Dose is defined as the total energy per unit area delivered to the substrate during an exposure of a target portion. Preferably the transmissivity shall be substantially stable during a plurality of exposures of adjacent target portions such as to avoid a necessity of intermediate dose calibrations. It is known that instability of transmissivity can occur due to, for example, an interaction between projection beam radiation and materials of optical elements of said radiation and projection system. Known transient variations in the transmissivity of said optical elements can be corrected for via a feed-forward control; see, for instance, U.S. Pat No. 9/461275, incorporated herein by reference.
For shorter wavelength radiation, especially for wavelength of 170 nm and below, absorption by air (through the presence of oxygen) becomes significant. Therefore, the optical path of the lithographic apparatus is evacuated or flushed (xe2x80x9cpurgedxe2x80x9d) with a gas (a xe2x80x9cpurge gasxe2x80x9d) transparent to the radiation used, commonly dry N2. In spite of the above known precautions, there is the problem of significant transmissivity variations due to, for instance, the presence of residual oxygen in said optical path, and leading to undesirable production errors.
One aspect of the present invention provides improved control over gas composition of gas traversed by the projection beam, so as to improve control over transmissivity variations of the radiation system and the projection system. In particular, it the invention may provide improved control over dose delivered to the substrate during an exposure of a target portion in a lithographic projection apparatus, particularly when radiation of wavelength less than about 170 nm is used.
This and other aspects are achieved according to the invention in a lithographic apparatus as specified in the opening paragraph, including a sensor to measure a gas composition in at least one region traversed by the projection beam; and a control responsive to said gas composition measured by said sensor to control the radiation energy delivered by said projection beam to said substrate during an exposure of a target portion.
Apparatus in accordance with the present invention may measure a gas composition in one or more regions through which the projection beam passes. The measurement results can serve as input for control means. The control can be arranged, for example, to calculate a prediction of absorption of radiation that will occur during an exposure of a target portion. With said prediction of absorption an appropriate correction to the dose can be calculated and adjustments to effectuate said correction can be applied. Said adjustments may comprise, for instance, an adjustment of the radiation power emitted by a radiation source supplying radiation to said radiation system, or, when said radiation source is an excimer laser, an adjustment of the number of pulses of radiation emitted by the radiation source during an exposure of a target portion. Input for the control means can also be, for example, measurement results representative of gas composition in a disk-shaped volume substantially comprising a pupil plane of the projection system (or a plane conjugate to said pupil plane in either the radiation system or the projection system). The control means can, in this example, be arranged to predict and adjust the angular distribution of radiation energy delivered to the substrate during an exposure of a target portion. The detection of gas composition preferably comprises measurement of the levels of gases known to absorb radiation of the wavelength of the projection beam, e.g. oxygen and water.
Conventionally, the dose delivered in an exposure is controlled by varying said radiation power or the duration of the exposure, or both. An energy sensor is provided at a convenient position in the radiation system, to measure the output of a radiation source supplying radiation to said radiation system. Said output as measured provides the basis for a feedback control and adjustment of, for instance, the radiation power emitted by the source or the exposure duration. Where an energy sensor is used to measure the output of the radiation source, the sensor means of the present invention may be arranged to measure gas composition in a region or regions downstream of the energy sensor. In this manner, the present invention can take account of absorption downstream of the energy sensor that would otherwise cause dose errors.
An adjustment determined according to the invention to be necessary to compensate for absorption by gas in the regions traversed by the projection beam can be combined with adjustments determined to be necessary to compensate for other factors, e.g. variations in the radiation source output or absorption by optical elements in the projection and/or radiation systems.
According to a further aspect of the invention, improved control over gas composition in a volume traversed by the projection beam is obtained by measuring said gas composition and by supplying an absorbent gas at a controlled concentration to said volume. Said absorbent gas serves to absorb radiation of the wavelength of said projection beam. A radiation absorbing system comprising a gas supply for supplying said absorbent gas may function as a partially transmissive optical filter, where the transmissivity can be varied by adjusting the gas composition.
The radiation absorbing system in a simple form comprises an enclosure having end faces substantially transparent to the radiation of the projection beam, e.g. made of CaF2, together with a supply of the absorbent gas connected to the enclosure via a control valve. A further valve controls the exit of gas from the region enclosed by the enclosure, which may be effected using a vacuum pump. For simplicity, an enclosure with end faces, as described above, may hereinafter be referred to as a xe2x80x9cchamberxe2x80x9d. The pressure and/or density of the gas in the chamber is controlled so as to provide the desired attenuation of the projection beam. The gas inlets and outlets are arranged so that the gas concentration/density in the chamber is uniform so that the beam is uniformly attenuated.
In more complex forms, the radiation absorbing system is arranged to provide a non-uniform concentration of absorbent gas so as to have a desired beam-shaping effect. This can be achieved with an array of individually controllable gas (micro-)jets allowing local control of the absorbent gas concentration or an arrangement of inlets and outlets configured to create an absorbent gas concentration gradient.
The positioning of the radiation absorbing system depends on the use to which it is put. If used for overall control of the dose (as defined above) it can be sited in, for example, the radiation system or, in case a laser is used as radiation source, in the laser beam provided to the radiation system. In the latter case it can be placed, for instance, relatively close to the radiation source. For filtering of diffraction orders of radiation diffracted upon traversing the mask, the radiation absorbing means can for instance be located such as to enclose a pupil plane of the projection system. For shaping the angular distribution of radiation energy of the projection beam the radiation absorbing means can be located such as to enclose a plane conjugate to said pupil plane, for instance in the radiation system. For control of intensity variations across the scanning slit, the radiation absorbing system can, for instance, be placed near the substrate or near the mask, or it can be placed such as to comprise a plane conjugate to the mask.
The volume to which the radiation absorbent gas is supplied may be free of other gases or may include other gases transparent to the radiation of the projection beam. For a projection beam with a wavelength of, for example, 157 nm, O2 may be used as the radiation absorbing gas whilst N2 may also be present as a non-absorbing purge gas.
Where a flow of radiation absorbing gas through the radiation absorbing means is provided, this can be used to carry away the heat generated on absorption of the radiation of the projection beam. Also, since the radiation beam traversing the absorbing gas may induce a chemical effect in said absorbing gas (and therefore induce a change of absorbance) the flow can be used to carry away the affected absorbing gas.
According to a further aspect of the invention there is provided a device manufacturing method comprising:
providing a substrate that is at least partially covered by a layer of radiation-sensitive material;
providing a projection beam of radiation using a radiation system;
using patterning means to endow the projection beam with a pattern in its cross-section;
projecting the patterned beam of radiation onto a target portion of the layer of radiation-sensitive material,
characterized by at least one of:
measuring the composition of gases in at least one region traversed by said projection beam;
controlling the intensity of said projection beam and/or the duration of an exposure in response to said measured gas composition so that a desired dose is delivered to said substrate during an exposure, and
supplying an absorbent gas at a controlled concentration to a volume traversed by said projection beam to effect a desired attenuation of said projection beam, said absorbent gas absorbing radiation of the wavelength of the projection beam.
Although specific reference may be made in this text to the use of the apparatus according to the invention in the manufacture of ICs, it should be explicitly understood that such an apparatus has many other possible applications. For example, it may be employed in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal display panels, thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms xe2x80x9creticlexe2x80x9d, xe2x80x9cwaferxe2x80x9d or xe2x80x9cdiexe2x80x9d in this text should be considered as being replaced by the more general terms xe2x80x9cmaskxe2x80x9d, xe2x80x9csubstratexe2x80x9d and xe2x80x9ctarget portionxe2x80x9d, respectively.
In the present document, the terms xe2x80x9cradiationxe2x80x9d and xe2x80x9cbeamxe2x80x9d are used to encompass all types of electromagnetic radiation, including ultraviolet radiation (e.g. with a wavelength of 365, 248, 193, 157 or 126 nm) and EUV (extreme ultra-violet radiation, e.g. having a wavelength in the range 5-20 nm).